The present invention relates to a method of plating and welding, and more specifically to a method for preventing the development of cracks below the seams during plating and welding.
During plating and welding of connections between low-alloy steels, fine cracks often occur in the heat-affected zones having a martensitic transformation structure. These zones are in the vicinity of the actual welding seam with the melt zone. Tests have shown that these cracks occur during stress relief annealing (approximately 600.degree. C.). This phenomenon is known and is called "stress relief cracking", which is disclosed in Document No. IIW-IX-648-69, and Document No. IIW-X-531-69 given at the 1969 Meeting of International Institute of Welding with the title "Stress-relief cracking in heat-affected zone" by H. Nakamura, T. Naiki and H. Okabayashi, page 10.
It is also known that the martensitic transformation structure in which the occurrence of cracks is observed during stress relief annealing can be avoided by slow cooling. In the above-cited publications it is further stated that the occurrence of cracks during stress relief annealing can be avoided if a short-time annealing treatment occurs at approximately 900.degree. C. after cooling and before the stress relief annealing so that the martensitic structure is refined by changing the size of its grains.
Both processes, however, cannot be used for welding structures without additional measures and do not bring about the desired result without additional restrictions during the heat treatment.
During a welding or plating treatment the material is heated locally to high temperatures so that rapid cooling occurs due to the heat dissipation to the surrounding areas. It is not the custom during such welding operations to provide slow cooling to the extent required to avoid crack formations. Otherwise the member would have to be heated to unduly high temperatures. Thus the individual parts are limited to preheating to temperatures of about a maximum of 200.degree. C.
Subsequent heating of the welding seam to temperatures above the stress relief annealing temperature is also not practiced. Such an operation would lead to the desired result only if the heating were very rapid. Moreover, heating of the entire member would reduce the strength of tempered steels to below the prescribed values.